Photovoltaic installations that connect to the power grid are formed by a set of photovoltaic panels (photovoltaic generator) and an electronic DC/AC converter, also called an inverter, that conditions the energy produced by the panels and injects it into the power grid.
This type of installation traditionally includes a low-frequency transformer between the converter and the power grid. This transformer provides galvanic insulation between the installation and the grid. Moreover, this transformer can be used to raise the output voltage of the converter.
The converter is made up of power transistors that switch at high-frequency to convert the direct current provided by the photovoltaic generator into alternating current that is injected into the power grid. This switching generates a variable voltage between the live parts and ground. Said voltage is called the common-mode voltage (VCM).
As can be seen in FIG. 1a, the photovoltaic generator has a parasitic capacity between the active terminals (positive and negative) and the ground (CPV), this parasitic capacity being proportional to the area and therefore to the power of the generator. On the other hand, the transformer has a parasitic capacity between the primary and secondary windings (CT). The whole system can be reduced to a simplified common-mode equivalent shown in FIG. 1b. 
Since the capacity of the transformer is much less than that of the photovoltaic generator, the common-mode voltage generated by the converter falls mostly on the transformer, therefore eliminating high-frequency voltage variations between the input terminals and ground. However, low-frequency transformers have several drawbacks, such as:                Large size        Large weight        Considerable increase in installation price        Reduced system output.        
Generally, large power facilities are formed by a set of DC/AC converters, each of which have their own transformer and photovoltaic generator, working in parallel to the same grid, as shown in FIG. 2a. Under these conditions, the whole system can be reduced to its simplified common-mode equivalent, shown in FIG. 2b. 
As can be seen, the system's behaviour in common mode can be studied as if it consisted of independent DC/AC converters. Therefore, each transformer supports the common-mode voltage, eliminating the variations in high-frequency voltage between the input terminals of the photovoltaic generator and ground.
One current solution to improve the output and reduce installation costs is to remove the transformer associated to each of the DC/AC converters, replacing it by a single transformer for the entire installation, as shown in FIG. 3a, using an isolated ground (IG) wiring scheme in the part of the DC/AC converters. This type of facilities improves the efficiency of the installation and reduces its cost. However, when removing the transformers associated to each converter a new common-mode circuit is created that connects all the DC/AC converters to a same equivalent point, resulting in the simplified diagram of FIG. 3b. 
This type of installations can produce undesired variations in high-frequency voltage between the input terminals and ground if the common-mode voltages of the different DC/AC converters are not the same. The occurrence of variations in high-frequency voltage between the input terminals and ground can damage the insulation of the photovoltaic generator, implying a risk for people. Moreover, the occurrence of these voltages generates problems of electromagnetic compatibility (EMC).
One alternative to solve this problem is to use a transformer with several primary windings to which each converter is connected, and a single secondary winding connected to the power grid, as shown in FIG. 4. In this case, the effect regarding the variation in high-frequency voltage between the input terminals and ground is similar to placing independent transformers in series with each converter. However, transformers of this type are not conventional, are difficult to manufacture and are more expensive.
Another alternative is the use of converters without a transformer, using modulation techniques to eliminate the variations in high-frequency voltage between input terminals and ground. However, these converters have lower output and present technical difficulties for achieving high power.